a) Field of the Invention
This invention relates to a valve system for an internal combustion engine, which valve system is suited for use in an automobile engine or the like of the type that engine valves, namely, intake and/or exhaust valves are designed variable in opening/closing timing and valve lift.
b) Description of Background Art
Mechanisms provided with plural kinds of cams of different profiles have been provided to date so that the valve timing and valve lift of each of intake and exhaust valves can be changed in an OHC (overhead camshaft) engine.
These mechanisms are constructed to obtain a valve opening/closing timing conforming with the state of operation of an engine, for example, by providing a camshaft with a high-speed cam and a low-speed cam and selectively achieving valve-driving states according to the profiles of the respective cams.
For example, FIGS. 3 to 7 illustrate a selection mechanism for a high-speed cam and a low-speed cam in such a mechanism. The selection mechanism is constructed as will be described next.
Interposed between cams 102,103,202 and valves 101,101 are rocker arms 104,105,204.
The cams 102,103,202 are constructed as shown in FIGS. 6 and 7. The cams 102,202 have a cam profile as low-speed cams, while the cam 103 has a cam profile as a high-speed cam.
In other words, the high-speed cam 103 is constructed to obtain a cam lift greater than the low-speed cams 102,202.
On the other hand, the rocker arms 104,105,204 are pivotally supported on a rocker shaft 106 and undergo rocking motion about the rocker shaft 106 in accordance with cam lifts by the cams 102,103,202.
Connection and disconnection between the low-speed rocker arms 104,204 and the high-speed rocker arm 105 are effected by pistons 107,108 and a stopper 109.
When working oil is fed to a side of a rear end of the piston 107 through oil passages 106a,204b in the state depicted in FIG. 4, the pistons 107,108 are driven leftwards in cylinders 204a,105a,104a as shown in FIG. 5 so that the low-speed rocker arms 104,204 and the high-sped rocker arm 105 are connected via the pistons 107,108.
When operation is carried out in this state, the low-speed rocker arms 104,204 and the high-speed rocker arm 105 both undergo large rocking according to the cam profile of the high-speed cam 103 having the greater cam lift.
As a consequence, the valves 101,101 are opened and closed at a valve timing defined by the high-speed cam.
When the pistons 107,108 are driven rightwards by a return spring 110 to the positions shown in FIG. 4, on the other hand, the low-speed rocker arms 104,204 are no longer in engagement with the high-speed rocker arm 105 and hence undergo small rocking according to the cam profile of the low-speed cams 102,202 having the smaller cam lift.
As a result, the valves 101,101 are opened and closed at a valve timing defined by the low-speed cams 102,202.
Each valve 101 is urged in a closing direction by a coil spring 33 via a valve stem 31 and a retainer 32. The retainer 32 and the coil spring 33 are arranged to avoid interference with the high-speed rocker arm 105 even when they assume highest positions.
Since the high-speed rocker arm 105 is undergoing large rocking for a high speed even when the low-speed rocker arms 104,204 are rocking at a small amplitude for a low speed, there is the potential problem that a rocking end portion of the high-speed rocker arm 105 may interfere with the coil springs 33. It is therefore constructed to avoid such interference by positioning an upper end of each coil spring 33 at a location the way down by a predetermined distance from an upper end of the valve stem 31.
Incidentally, a return spring 111 is mounted on a main body of an engine so that the high-speed rocker arm 105 is urged upwards and is maintained in contact with the high-speed cam 103.
As a valve system equipped with high-speed and low-speed cams so that valve driving states according to the profiles of the respective cams can be selectively attained to obtain a valve opening/closing timing in conformity with the state of operation of an engine, a construction such as that illustrated in FIGS. 8(A) to 12 can also be considered with a view to providing valve-operating characteristics improved over those of the above-described valve system.
Intake or exhaust valves 2,3 arranged in a pair are driven by a cam 12 or 13 by way of a main rocker arm 24 and subrocker arms 26,15.
In the illustrated example, the subrocker arms 26,15 are provided with a low-speed roller 18 and high-speed roller 19 which are maintained in contact with the cams 12,13, respectively, whereby the subrockers 26,15 receive drive force through these rollers 18,19. The low-speed roller 18 and high-speed roller 19 are rotatably supported via bearings 18B,19B on shafts 18A,19A attached to the subrocker arms 26,15, respectively.
The cam 12 is provided with a cam profile for low-speed valve timing like the above-described cams 102,202 while the cam 13 is equipped with a cam profile for high-speed valve timing like the above-described cam 1031 (see FIGS. 6 and 7).
The main rocker arm 24 is pivotally supported on a cylinder head 1 via a rocker shaft 16. Further, proximal end portions of the subrocker arm 26,15 are loosely fitted in the main rocker arm 24. Interposed between the main rocker arm 24 and the subrocker arms 26,15 are hydraulic piston mechanisms 27,17 as mode change-over means.
The hydraulic piston mechanisms 27,17 are constructed in such a way that, upon feeding of a predetermined hydraulic pressure, either a piston 17A or a piston 27A is caused to project from the main rocker arm 24 into the corresponding subrocker arm 15 or 26.
The projecting piston 17A or 27A then engages the corresponding subrocker arm 15 or 26 so that the subrocker arm 15 or 26 is associated with the main rocker arm 24 via the piston 17A or 27A so engaged. The main rocker arm 24 can hence be driven by the cam 13 or 12 which has been brought into engagement with the subrocker arm 15 or 26.
In the above example, as illustrated in FIGS. 8(A) to 12, the pistons 17A,27A in the hydraulic piston mechanisms 17,27 are accommodated within bores 17C,27C formed in the rocker shaft 16 and are urged by springs 17B,27B in predetermined directions, respectively. Described specifically, the piston 17A of the hydraulic piston mechanism 17 is urged in the direction that the subrocker arm 15 does not engage the main rocker arm 24 (i.e., downwards as viewed in FIG. 9), while the piston 27A of the hydraulic piston mechanism 27 is urged in the direction that the subrocker arm 26 engages the main rocker arm 24 (i.e., upwards as viewed in FIG. 10). Oil compartments 17G,27G are arranged to produce hydraulic pressures in opposition to these urging forces, respectively. The hydraulic piston mechanism 17 remains in a non-engaged position as long as no hydraulic pressure is fed to the oil compartment 17G. As soon as the oil compartment 17G is fed with a hydraulic pressure, the hydraulic piston mechanism 17 is brought into an engaged position. The hydraulic piston mechanism 27 remains in an engaged position as long as no hydraulic pressure is fed to the oil compartment 27G. As soon as the oil compartment 27G is fed with a hydraulic pressure, the hydraulic piston mechanism 27 is brought into a non-engaged position.
Formed in communication with the bores 17C,27C are oil holes 17D,27D for feeding lubeoil to the low-speed roller 18 and the high-speed roller 19, respectively. Further, openings of the holes 17C,27C are closed by caps 17E,27E, respectively.
By changing over the hydraulic piston mechanisms 17,27 as described above, either the cam profile of the cam 12 for low-speed valve timing or the cam profile of the cam 13 for high-speed valve timing is selected to achieve a desired valve timing in correspondence to the state of operation of the engine.
The rocker shaft 16 is pivotally supported on bearing portions 1A (see FIG. 8(B)). An oil passage 16A is formed inside the rocker shaft 16.
Also arranged is a lost motion mechanism 20 which pushes a lever portion 15C of the subrocker arm 15. The lost motion mechanism 20 is provided with an outer casing 20A and an inner casing 20B which can advance or retreat in an axial direction relative to the outer casing 20A. By a spring 20C accommodated in a space 20E between both the casings 20A and 20B, the inner casing 20B is urged to project downwardly. The outer casing 20A is fixedly connected to a lost motion holder 1B. A free end portion of the inner casing 20B, which is urged to project as described above, is provided with a contact portion 20D maintained in contact with the lever portion 15C of the subrocker arm 15.
Such a valve system is however accompanied by such problems as will be described next.
The valves 2,3 are each urged in a closing directions by an associated coil spring 4, whose outer shape is cylindrical, via a corresponding valve stem 2A or 3A and a retainer 5. It is therefore necessary to provide the retainer 5 and coil spring 4 in such a way that they do not interfere with the corresponding subrocker arm 26 or 15 even when they assume highest positions.
In the above valve system, the valves 3,2 can be made inoperative or can be allowed to rest by bringing each of the hydraulic piston mechanisms 17,27 into the non-engaged position. Despite the valves 3,2 not moving in such a mode, the subrocker arms 15,26 are continuously driven by the cams 13,12, resulting in the potential problem that, as shown by way of example in FIG. 12, the subrocker arm 15 may interfere with the coil spring 4 and/or the retainer 5. Although not illustrated in any figure, there is also the potential problem that the subrocker arm 26 may interfere with the coil spring 4 and/or the retainer 5. With respect to each of the valves 3,2, it is therefore necessary, as shown in FIG. 11, to design the valve stem 3A in such a way that a sufficient distance (see the dimension A shown in FIG. 11) can be left between an upper end of the coil spring 4 and an upper end of the valve stem 3A.
In particular, there is the high potential problem that a rocking end portion 15A of the high-speed subrocker arm 15 may interfere with the coil spring 4 of the valve 3 because the high-speed subrocker arm 15 is undergoing large rocking for a high speed. The upper end of the coil spring 4 is therefore positioned at a point substantially lower than the upper end of the valve stem 3A, whereby the potential problem of interference can be avoided.
Even when the valves are not made inoperative, there is the possibility that the high-speed subrocker arm 15 undergoes large rocking for a high speed by the cam 13 while the valve 3 is reciprocated at a small stroke for a low speed. In this case, there is also the potential problem that the subrocker arm 15 may interfere with the coil spring 4 and/or the retainer 5. It is hence necessary to leave the distance A as much as needed between the upper end of the coil spring 4 and the upper end of the valve stem 3A.
Further, the high-speed subrocker arm 15 may jump off from the high-speed cam 13 during high-speed operation. Even in such a case, it is also necessary to ensure the avoidance of interference between the rocking end portion 15A of the high-speed subrocker arm 15 and the coil spring 4.
As has been described above, it is necessary for each of the valve stems 2A,3A to have a sufficient distance A (see FIG. 11) between the upper end of the coil spring 4 and the upper end of the valve stem. Especially where there is the high-speed rocker arm 15, it is necessary to increase the dimension A compared with a construction in which the high-speed rocker arm 15 is not provided. This leads to an increase in the overall height of the valve system. As a result, the overall height (the dimension C shown in FIG. 11) of the valve system becomes greater, leading eventually to an engine having a larger height or width. Although it is basically desired to promote the dimensional reduction of engines, use of such a valve system conversely increases the engine dimensions.
Such a problem similarly arises on the first-mentioned conventional valve system.
First, in the structure of the conventional valve system shown in FIGS. 3 to 6, the high-speed rocker arm 105 is undergoing large rocking for a high speed even when the low-speed rocker arms 104,204 are rocking at a small amplitude for a low speed. There is accordingly the potential problem that the rocking end portion of the high-speed rocker arm 105 may interfere with the coil springs 33. It is therefore necessary to extend the end portions of the valve stems 31 by a predetermined length so that the valve system is disposed above the upper end of the coil spring 33 to avoid the interference.
Accordingly, it is necessary to leave a sufficient distance (see the distance A shown in FIG. 11) between the upper end of each valve stem 31 and the upper end of the associated coil spring 33. Especially where there is the high-speed rocker arm 105, it is necessary to increase the dimension A compared with a construction in which the high-speed rocker arm 105 is not provided. This leads to an increase in the overall height (see the dimension C depicted in FIG. 11) of the valve system.
This unavoidably leads to an engine having a greater overall height, resulting in problems such that the freedom of vehicle mountability is lowered, the weight of the engine assembly is increased, and the moving performance of a vehicle is deteriorated.
To avoid the interference, one could consider reducing the overall length of each coil spring 33. Such a reduced overall length however requires an additional measure for ensuring production of predetermined urging force. The freedom of design is hence lowered and modifications may become necessary in the whole engine design. Use of such shorter coil springs is therefore not preferred from the standpoints of cost, engine performance and the like.
As has been described above, a valve system in which the motion of rocker means for driving valves may become greater than the motion of the valves requires a consideration so that interference between the side of the valves and the side of the rocker means can be prevented. This requirement therefore leads to the problem that the freedom of vehicle mountability is lowered, the weight of the engine assembly is increased, and the moving performance of a vehicle is deteriorated.
Techniques making use of valve springs in a tapered or barrel-shaped form as viewed in cross-sections taken along their axial center lines are disclosed, for example, in Japanese Utility Model Application Laid-Open (Kokai) No. SHO 60-38107 and Japanese Utility Model Application Laid-Open (Kokai) No. SHO 60-88011. These conventional techniques are however intended to reduce the inertial mass and dimensions by making the springs smaller. They do not contain any disclosure about so-called free rocker arms which do not directly drive intake or exhaust valves. The techniques therefore do not teach anything about the interference between the free rocker arms and the springs.
For reducing the overall height of the valve system, it is desired to achieve the avoidance of interference between the low-speed subrocker arm 26 and the coil spring 4 while allowing the coil spring 4 to extend into the range of rocking motion of the low-speed subrocker arm 26.